1. Field of the Invention
The invention relates to a belt strap for safety belts for vehicles, particularly motor vehicles. The belt strap should have as low friction an outer surface as possible so that it can run through the safety belt system, i.e. the coiling device, past the guide fitting at the top of the bodywork and past the insertion fitting at the bottom with as little friction as possible. Such straps are also referred to in the technical terminology as "low-friction straps". The lower the friction the higher is the proportion of the force of the coiling spring which may be used for tensioning and coiling. With belt straps of very low friction a weaker spring can be provided than is otherwise usual. Less pressure is advantageously exerted by it on the body of the person secured than by previously usual springs. The wearing comfort of the safety belt is thus increased.
A lower friction of the belt straps is also of importance at low outdoor temperatures. The coiling spring which comprises spring steel becomes harder at lower temperatures and can only exert a lesser tension on the belt strap. The belt strap also becomes harder and performs more flexural work at the point where it must bend, principally around the guide fitting. This can result in the belt strap not being completely wound up when it is taken off so that a portion of the belt strap hangs out of the door opening. If the driver does not notice this he can shut a loop of the belt strap in the door. The belt strap can be damaged by this, particularly when it is pinched in the region of the door lock. On the other hand, there is the danger that after taking off the belt strap the driver treads into a hanging loop of the belt strap when getting out and thus damages the belt strap and possibly also injures himself.
2. Description of Related Art
It is known to make belt straps of low friction by applying a coating (Avivage) namely a chemical product which adheres to the outer surface of the belt strap and reduces the friction. The disadvantage of this known method is that the coating is partially worn away after some time of use and no longer fulfils its purpose of making the strap of low friction. The coating can also be destroyed by chemical environmental effects.
Belt straps which are supplied for use in motor vehicles must pass an abrasion test which is determined by U.S. Standard MVSS, No. 209 (para. 571.209). (Published in USA Federal Register, Volume 36, No. 232, dated Dec. 2, 1971). In accordance with this Standard a belt strap is firstly subjected to an abrasion test. It is subsequently stretched to breaking and the breaking load is determined. This must be at least 75 to 85% of the original breaking load. The belt strap is otherwise not permitted by the competent national authorities to be used as a safety belt in motor vehicles.
A belt strap which has lost its coating is weakened by abrasion in an impermissibly short time to the extent that it no longer has the necessary minimum breaking load. The safety of the user is then no longer guaranteed.
Belt straps are known whose filaments have 8 to 14 dtex. In order that these pass the abrasion test either a coating is necessary, which has the disadvantages referred to above, or a twining of 80 to 100 twists per meter.